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For the vast majority of machining applications, liquid coolants are the preferred method of preventing heat damage in both workpieces and cutting tools. However, many industries have rigorous standards for part cleaning that makes both oil- and water-based coolants less attractive, and some workpieces must be machined dry to avoid contamination from the coolants. According to Erik Poulsen, Medical Market Segment Manager for GF Machining, one alternative medical manufacturers should look at is supercritical carbon dioxide (supercritical CO₂).

Lubrication without Residue

The first question I had when I spoke to Poulsen was simple: What does supercritical mean? “Supercritical refers to a state when a substance is at such an extreme pressure that there ceases to be a difference between the substance’s liquid and solid phases,” Poulsen says. “It behaves like a liquid and like a gas simultaneously.”

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For carbon dioxide, the supercritical phase is at around 1,500 psi and 25.6 degrees Celsius (78 degrees Fahrenheit). “Once it’s released, it rapidly evaporates and cools to about -70 degrees Celsius,” Poulsen explains. These properties have supercritical CO₂ seeing widespread use in cleaning electronics, as water can damage the final product. However, its machining applications — sometimes referred to as cryogenic machining — are less widely known.

When used in machining, tiny amounts of oil get injected into the supercritical CO₂ fluid, dispersing evenly. “This creates miniature spheres of oil throughout the fluid that dramatically improve cutting conditions,” Poulsen says, explaining that the oil evaporates along with the carbon dioxide. This is called minimum quantity of lubrication (MQL), and its benefits are more clearly seen in difficult materials like titanium.

“Titanium is one of the materials where we see the greatest success [with supercritical CO₂],” Poulsen says. Machining titanium generates a lot of heat, and managing that heat is key to successful part production, something supercritical CO₂ can handle better than many options. “In our tests machining titanium, using supercritical CO₂ with an MQL additive doubles the material removal rate and doubles the life of the cutting tool.” That material removal rate translates to a 30-40% decrease in production time, according to Poulsen.

While that decrease in production time is enticing on its own, this process also delivers energy savings. “Machining something in 30% less time reduces energy consumption per part by roughly 30%,” Poulsen explains. “But there’s even more energy savings because you can the energy use of flood coolant pumps, increasing total energy savings to around 45%.”

One field that may be particularly interested in these benefits is medical machining, both because titanium is a common material in medical implants and because eliminating coolant residue it much easier to prevent contamination in the final product. 

Under Pressure

Some readers may have already guessed one of the major drawbacks of supercritical CO₂: 1,500 psi is a lot of pressure, enough to damage machines not specially designed for this process. Additionally, it often uses through-spindle coolant, which means using through-coolant cutting tools and equipping the spindle with a rotary union capable of handling 1,500 psi and speeds up to 30,000 rpm.

GF Machining, however, has been developing a turnkey solution with through-coolant systems capable of handling the intense pressures. “GF is the only milling machine provider developing a machine with supercritical CO₂ fully integrated, so you don’t have to take the risk of developing the process yourself,” Poulsen says. “We know how to do it, and we can put it together for you.”

The company has also tested the process with a number of different cutting tools, finding that a variety of milling tools can benefit from the process. “We’ve done tests down to tool diameters of 1 millimeter, and it holds and cools,” Poulsen says. “It doesn’t get the same bump in tool life and speed, but it still eliminates flood coolant.”

While a turnkey solution for machining with supercritical CO₂ coolant may not be available today, the technology exists, and machine shops plugged into the medical manufacturing industry may want to keep it in mind as they plan future work.